When compared with the conventional optical repeater having 3R (Reshaping, Retiming, Regenerating) functions known heretofore, the optical fiber amplifier has desirable features such as independency on the transmission rate, susceptibility to simplified implementation of the repeater, possibility of implementation with large capacity owing to the wavelength multiplexing capability and others. Thus, the optical fiber amplifier is expected to promise a key component which is capable of enhancing flexibility of the optical communication system. In particular, in an optical network in which the wavelength multiplexing technique is adopted, it is possible to achieve remarkable economization.
As the conventional or prior art optical amplifying repeater apparatus of the type mentioned above, there can be mentioned, for example, the one disclosed in “OPTICAL AMPLIFIERS AND THEIR APPLICATION”, PP. 280-283, 1998. FIG. 12 is a block diagram showing the prior art optical amplifying repeater apparatus disclosed in the above-mentioned publication.
In FIG. 12, reference numerals 1; 4 denote optical amplifiers each of fixed gain type for amplifying en bloc light signals of wavelengths λ1 to λn, numeral 3 denotes an adjustable optical attenuator, numeral 77 denotes an optical branching device for extracting a part of output power, numeral 6 denotes an optical attenuator control circuit for controlling the adjustable optical attenuator, numerals 501; 506 denote optical amplifiers, respectively, each implemented by making use of an erbium-doped fiber or the like, numerals 502 and 507 denote pumping light sources, respectively, numerals 503, 504, 508 and 509 denote optical branching devices for extracting parts of power of the light signals inputted thereto, respectively, and reference numerals 505 and 510 denote pumping light source control circuits for controlling the pumping light sources, respectively.
Next, description will be made of operation of the optical amplifying repeater apparatus. The wavelength-multiplexed light signals λ1 to λn as inputted are first amplified by the fixed-gain optical amplifier 1 with a predetermined gain G0 and subsequently undergo attenuation with a predetermined attenuation factor through the adjustable optical attenuator 3. The wavelength-multiplexed light signals outputted from the adjustable optical attenuator 3 are again amplified by the fixed-gain optical amplifier 4 with a predetermined gain G1 to be ultimately outputted by way of the optical branching device 77. In that case, a part of the output signal is extracted through the optical branching device 77 and detected by the optical attenuator control circuit 6, which circuit is so designed as to control the factor of attenuation effectuated by the adjustable optical attenuator 3 so that the part of the output light signal extracted through the optical branching device 77 assumes a predetermined value. In this manner, the overall or total output power of the optical amplifying repeater apparatus is maintained at a constant value. In the case where the number of the wavelengths is constant, the output powers of the respective wavelengths can be maintained constant on a wavelength-by-wavelength basis, rendering it possible to realize ideal operation.
At this juncture, operation of the fixed-gain optical amplifier 1 will be described in detail. Input/output powers to/from the fixed-gain optical amplifier 1 are monitored through the optical branching devices 503 and 504, respectively, wherein the pumping light source control circuit 505 controls the pumping light source 102 such that the ratio between the input and output powers of the fixed-gain optical amplifier can be maintained to be constant. In this way, the gain of the fixed-gain optical amplifier 1 is held constant. Similar operation is performed for the fixed-gain optical amplifier 4 as well.
Furthermore, FIG. 13 shows in a block diagram another prior art optical amplifying repeater apparatus which is disclosed, for example, in “OPTICAL AMPLIFIERS AND THEIR APPLICATIONS”, MD1, 1998. This optical amplifying repeater apparatus is so arranged as to perform not only amplification of the light signals of wavelengths λ1 to λn but also gain control for the optical repeater on the basis of monitoring information carried by a monitoring light signal λs sent from terminal equipment. In FIG. 13, reference numeral 11 denotes an optical branching device for separating the monitoring light signal λs from the light signals of wavelengths λ1 to λn, and reference numeral 17 denotes a monitoring light receiver.
Next, description will turn to operation of the optical amplifying repeater apparatus described above. The wavelength-multiplexed light signals λ1 to λn as inputted are first amplified by the fixed-gain optical amplifier 1 with a predetermined gain G0 to subsequently undergo attenuation with a predetermined attenuation factor through the adjustable optical attenuator 3. The wavelength-multiplexed light signals outputted from the adjustable optical attenuator 3 are again amplified by the fixed-gain optical amplifier 4 with a predetermined gain G1 to be outputted via the optical branching device 77. A part of the output signal is extracted through the optical branching device 77 and detected by the optical attenuator control circuit 6, which circuit is also so designed as to control the attenuation effectuated by the adjustable optical attenuator 3 so that the part of the output signal extracted through the optical coupling device 77 assumes a predetermined value. In this manner, the overall total output power of the optical amplifying repeater apparatus is maintained at a constant value or level. So long as the number of the wavelengths is constant, the output powers of the respective wavelengths can be maintained constant on a wavelength-by-wavelength basis, whereby ideal operation can be ensured. The information about the number of wavelengths is contained in the monitoring information carried by the monitoring light signal λs sent out from the terminal equipment and thus inputted to the optical attenuator control circuit 6 after reception by the monitoring light receiver 17.
In the optical amplifying repeater apparatus of the structures described above, a part of the total output power is extracted by the optical branching device 77 for the purpose of controlling the adjustable optical attenuator 3 so that the output power can be maintained to be constant. As a result of this, very troublesome procedure is required for coping with increase or decrease of the number of wavelengths. More specifically, because the total output power of the optical amplifying repeater apparatus depends on the number of wavelengths, there arises necessity of messaging in advance to the optical attenuator control circuit 6 the value which the light power extracted through the optical branching device 77 is to assume, when the number of wavelengths is changed. Consequently, in the case where one of the wavelength-multiplexed light signals of wavelengths λ1 to λn is not transmitted due to some failure in sender equipment, by way of example, the messaging procedure such as mentioned above will not be in time for coping with the change of the number of wavelengths, thus bringing about corresponding changes in the powers of the other wavelengths, which of course will exert adverse influence to the quality of communication.
An object of the present invention is to solve the problem such as mentioned above and to provide an optical amplifying repeater apparatus which can positively protect the quality of communication from being degraded even when the powers of the other wavelengths change and further provides an optical amplifying/repeating transmission system which can maintain the system gain to be constant independently of the light signal power as inputted or the number of wavelengths thereof.